不匹配的dispatch_group_enter和dispatch_group_leave会发生什么？

我有一个异步NSOperation下载多个ImageFile对象的数据。由于这一切都是异步发生的，我使用一个调度组跟踪请求，然后在完成操作后完成操作。不匹配的dispatch_group_enter和dispatch_group_leave会发生什么？

我的问题是当操作过早结束时发生的情况，无论是通过取消还是通过其他错误。调度组将保留无与伦比的dispatch_group_enter和dispatch_group_leave，因此不会调用dispatch_group_notify。系统永远在等待系统保留的区块吗，还是在NSOperation发布后会被释放？

或者是我的方法不理想，我该怎么做呢？

- (void)main 
{ 
    if (self.cancelled) { 
     [self completeOperation]; 
     return; 
    } 

    NSManagedObjectContext *context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; 
    context.persistentStoreCoordinator = self.persistentStoreCoordinator; 
    context.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy; 

    [context performBlock:^{ 

     NSFetchRequest *request = [ImageFile fetchRequest]; 
     request.predicate = .... 
     request.sortDescriptors = ... 

     NSError *error; 
     NSArray *imageFiles = [context executeFetchRequest:request error:&error]; 
     if (!imageFiles) { 
      // Error handling... 
      [self completeOperation]; 
      return; 
     } 

     dispatch_group_t group = dispatch_group_create(); 

     for (ImageFile *imageFile in imageFiles) { 
      dispatch_group_enter(group); 
      @autoreleasepool { 
       [self.webService requestImageWithId:imageFile.id completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) { 

        if (self.cancelled) { 
         [self completeOperation]; 
         return; 
        } 

        [context performBlock:^{ 
         if (data && !error) { 
          imageFile.data = data; 
          NSError *error; 
          if (![context save:&error]) { 
           // Error handling... 
           [self completeOperation]; 
           return; 
          } 
         } 
         dispatch_group_leave(group); 
        }]; 
       }]; 
      } 
     } 

     dispatch_group_notify(group, dispatch_get_main_queue(), ^{ 
      [self completeOperation]; 
     }); 
    }]; 
}

来源

2017-02-15 trapper